Amphibolite

Amphibolite is a dark, dense, medium- to coarse-grained metamorphic rock composed mainly of amphibole (usually hornblende) and plagioclase feldspar, with little or no quartz. It often shows a weak foliation or lineation defined by aligned hornblende needles, giving it a streaky black-and-white appearance.

Frequently it carries accessory minerals such as garnet, epidote, or biotite, which add red or green flecks. As one of the most common metamorphic rocks of the deeper continental crust, amphibolite records medium- to high-grade conditions and is widely used as a durable building and decorative stone.

It is found in metamorphic terrains and shield regions around the world.

Amphibolite forms by the medium- to high-grade regional metamorphism of mafic igneous rocks such as basalt and gabbro (forming ortho-amphibolites) or, less commonly, of marls and impure dolomitic sediments (para-amphibolites). Under these conditions, the original minerals recrystallize into hornblende and plagioclase.

The rock forms at amphibolite-facies conditions, roughly 500-750 degrees C and moderate pressures, typical of the middle and lower continental crust in mountain belts. Directed pressure aligns the hornblende crystals, producing foliation. Amphibolites are widespread in Precambrian shields and orogenic belts, including the Canadian Shield, Scandinavia, and the Alps.

Amphibolite is dark green to black, dense, and often shows a streaky or weakly banded fabric from aligned needle-like hornblende crystals. Look for the elongate black amphibole grains together with paler plagioclase, and sometimes red garnet porphyroblasts.

It has a hardness around 5-6 and a vitreous luster on fresh hornblende faces. Distinguish it from basalt, which is much finer grained and not foliated, and from gabbro, which lacks foliation and contains pyroxene rather than hornblende. Eclogite is denser and contains red garnet in green omphacite. The combination of abundant hornblende, plagioclase, weak foliation, and dark color is diagnostic.

Amphibolite is widely used as a construction and dimension stone for building facings, tiles, paving, and countertops, often marketed as 'black granite,' and as crushed aggregate for roads and concrete because of its toughness.

Its dark, durable character makes it popular for monuments and decorative cladding. For geologists it is an important indicator of metamorphic grade and of the former presence of basaltic rocks. Some Stone Age cultures used amphibolite for polished axes. It has minimal metaphysical following, its main value being practical and scientific.